1. Field of the Invention
The present invention relates to a spatial correction device for an image analyzer.
2. Description of the Prior Art
By spatial correction is understood the correction of defects, due to the analyzer and which result in a spatial distortion between the analyzed image and the image restituted by the analyzer. By way of example, defects of convergence, geometry or shading may be mentioned.
Convergence defects only occur in color image analyzers. They are due to imperfect superimposition, on the image restitution support medium, of the electron beams associated with each of the three fundamental colors and they result in the presence of colored fringes on the outlines of objects.
Geometry and shading defects occur in all image analyzers, whether they operate in black and white or in color and are due to imperfections of the analysis system itself. Geometry defects result in geometrical deformation of the restituted image in relation to the analyzed image, and shading defects result in the presence of a background of non constant gray in the restituted image when the analyzed image has a background of a constant gray.
A device for correcting convergence defects for color cameras is disclosed in U.S. Pat. No. 4,437,110, issued Mar. 13, 1984, filed in the name of the Applicant. This device comprises essentially a memory, called a correction memory, adapted to store, for each of the rectangles resulting from dividing the screen of the camera into a grid of M groups of L lines each and N columns, a predetermined horizonntal scan correction signal and a predetermined vertical scan correction signal proper to each of the two of the three fundamental colors, and a sequencer for controlling the application of these correction signals to scanninng means during passage of the scanning means through the corresponding rectangles of the screen. Means for attenuating the discontinuities existing between horizontal and vertical scan correction signals associated with adjacent lines of the same column are also provided so as avoid the lines from moving closer together or moving away too suddenly during transitions between the different values of the corresponding correction signals. Two examples of these attenuation means are given. One analog embodiment consists in attenuating these discontinuities by means of filters of the "RC" type. One digital embodiment consists in attenuating these discontinuities by causing integration of the correction signal by means of an adder and a memory, called integration memory, associated with the correction memory.
Though these embodiments of means for attenuating the discontinuities between correction signals contribute in preventing the lines from suddenly drawing closer or moving away, they leave untouched another problem which is due to the presence of derivative ruptures during transitions between correction signals. In fact, the method of filtering by integration only attenuates the amplitude difference between two successive correction signals, but leaves fronts to continue to exist during transitions between successive correction signals. The method of filtering by means of "RC" type filters attenuates both the amplitude difference and the fronts between successive correction signals. However, it does not attenuate these fronts perfectly, for it maintains the presence of derivative ruptures during transitions.
These derivative ruptures have harmful consequences on the behavior of some component elements of the analyzer. The aim of the present invention is to avoid these disadvantages.